1. Field of the Invention
The present invention generally relates to television signal distributor apparatuses, receiver apparatuses, and television signal transmission systems and methods. More specifically, the present invention relates to a television signal distributor apparatus, a receiver apparatus, a television signal transmission system and method, which are particularly suitable for implementing a small-scale television signal distribution network in which television signals received via a master antenna provided on the rooftop at collective housing, such as an apartment building, are delivered to each household via a cable.
2. Description of the Related Art
In collective housing such as an apartment building, there is usually provided a television signal transmission system in which television signals received via a master antenna provided on the rooftop are delivered to each household via a cable. In VHF-band and UHF-band terrestrial broadcasting services, broadcasting stations are allocated with respectively different frequency bands. The television signal transmission system mixes signals received via a VHF antenna and a UHF antenna while adjusting the gain thereof, and then delivers the mixed signals to each household via a single cable.
Recently, BS (broadcasting satellite) broadcasting services are available as well as VHF-band and UHF-band terrestrial broadcasting services. BS broadcasting signals are transmitted on electromagnetic waves in the 12-GHz band, and the signals received via a parabola antenna are converted into intermediate frequency signals in the 1-GHz band in an LNB (Low Noise Block Down Converter) provided in association with the parabola antenna. The BS intermediate frequency signals in the 1-GHz band do not overlap the terrestrial broadcasting signals in the VHF and UHF bands. Thus, it is feasible to mix the 1-GHz band BS intermediate frequency signals and the VHF-band and UHF-band terrestrial broadcasting signals so that the mixed signals are delivered to each household via a single cable.
Furthermore, with the increasing popularity of CS broadcasting services, it is desired that a television signal transmission system which delivers CS broadcasting signals via a cable to each household be provided in collective housing such as an apartment building.
However, CS broadcasting signals are broadcast using the 12-GHz band and CS intermediate frequency signals are in the 1-GHz band, which overlaps the band allocated for BS intermediate frequency signals. Thus, it is not feasible to simply mix the CS intermediate frequency signals, the BS intermediate frequency signals, and the VHF-band and UHF-band terrestrial broadcasting signals and to deliver all these signals to each household via a single cable.
More specifically, BS broadcasting signals and CS broadcasting signals are both broadcast using the same 12-GHz Ku band, the local oscillation frequencies of LNBs for BS broadcasting signals and CS broadcasting signals being, respectively, 10.678 GHz and 11.2 GHz. Furthermore, CS broadcasting signals includes both vertically polarized signals and horizontally polarized signals. Thus, there exists an overlap between the frequency band BS-IF for the BS intermediate frequency signals and the frequency band CS-IF for the CS intermediate frequency signals. Accordingly, it is not feasible to simply mix the CS intermediate frequency signals, the BS intermediate frequency signals, and the VHF-band and UHF-band terrestrial broadcasting signals, and to deliver all these signals to each household via a single cable.
In view of the above, in conventional television signal transmission systems which are made compatible with CS broadcasting signals, CS intermediate frequency signals are frequency-shifted into the frequency bands above the frequency band for BS intermediate frequency signals.
More specifically, when BS intermediate frequency signals and the VHF-band and UHF-band terrestrial broadcasting signals are mixed for transmission via the same cable in the conventional television signal transmission systems, the frequency allocation of the mixed signals is as shown in FIG. 2. Referring to FIG. 2, the frequency allocation includes the VHF low band A11, the VHF mid band A12, the VHF high band A13, the VHF super high band A14, the UHF band A15, and the frequency band A16 for BS intermediate frequency signals. When CS intermediate frequency signals are also mixed with the BS intermediate frequency signals and the VHF-band and UHF-band terrestrial broadcasting signals for transmission via the same cable, frequency bands A17 and A18 above the frequency band A16 for BS intermediate frequency signals are allocated respectively for vertically polarized signals and horizontally polarized signals of the CS intermediate frequency signals. The vertically polarized signals and the horizontally polarized signals of the CS intermediate frequency signals in the 1-GHz band B11 and B12 are frequency-shifted into the frequency bands A17 and A18.
As described above, by frequency-shifting CS intermediate frequency signals so that the CS intermediate frequency signals are allocated in the frequency bands A17 and A18 above the frequency band A16 for BS intermediate frequency signals, the frequency band A16 for BS intermediate frequency signals and the frequency bands A17 and A18 for CS intermediate frequency signals do not overlap, allowing the VHF-band and UHF-band terrestrial broadcasting signals, the BS intermediate frequency signals, and the CS intermediate frequency signals to be transmitted via a single cable.
However, some broadcasting services employ two satellites for broadcasting, for example, the Japanese SKY PerfecTV. In that case, vertically polarized signals and horizontally polarized signals from each of the two satellites are transmitted using the same carrier frequency.
Thus, if the CS intermediate frequency signals are frequency-shifted into a frequency band above the frequency band for the BS intermediate frequency signals, the frequency allocation extends up to an extremely high frequency in order to accommodate the vertically polarized signals and horizontally polarized signals from each of the two satellites.
More specifically, a single band for CS intermediate frequency signals requires a bandwidth on the order of 500 MHz. Thus, in order to accommodate all the four signals, the required bandwidth is on the order of 500 MHz×4=2000 MHz. If the 2,000-MHz is allocated above the frequency band for BS intermediate frequency signals, i.e., 1,035 MHz to 1,335 MHz, the frequency allocation extends up to as high as approximately 3,500 MHz.
The frequency range compatible with the current transmission apparatuses is up to approximately 2,150 MHz; thus, compatibility with the frequency range of up to 3,500 MHz poses a technical difficulty of developing new apparatuses. In addition, implementation of such a system in a wide area involves sophisticated design techniques which are beyond the techniques of ordinary works.
It may be technically possible to provide a separate cable for transmission of CS intermediate frequency signals in addition to a cable for transmission of terrestrial broadcasting signals in the VHF and UHF bands and BS intermediate frequency signals, so that the CS intermediate frequency signals can be transmitted to each household even if the CS broadcasting service employs two or more satellites. However, the approach requires the complex work of providing new cables in existing collective housing, which is quite costly. In addition, if separate cables are provided for terrestrial broadcasting signals in the VHF and UHF bands and BS intermediate frequency signals, and for CS intermediate frequency signals, respectively, an additional equipment for switching between the cables is required at each household.